Dimethyl sulfoxide, also known in the industry as DMSO, is a dipolar aprotic solvent which is useful for dissolving both ionic and covalent compounds. DMSO dissolves ionic salts, such as sodium chloride or potassium bromide, as well as many nonionic compounds of low molecular weight. When using DMSO as a solvent for both inorganic salts and organic compounds in a single operation, the standard method for separating the organic compound from the DMSO and salts is to add water to the solution. The DMSO and salts enter the water phase, while the organic material forms a second phase which can be easily separated by physical means.
The resulting DMSO/inorganic salt/water phase can present a difficult disposal problem. In addition, DMSO is a fairly expensive solvent and thus, recovery would provide an economic incentive as well as overcome the problem of disposal.
In other situations, DMSO may be contaminated with inorganic salts alone. Again, purification of the DMSO by removal of the soluble inorganic salts may be necessary.
Berg, in U.S. Pat. No. 5,599,979 discloses the use of extractive distillation, which separates two compounds with similar boiling points by adding a higher boiling substance, to separate formic acid from acetic acid.
In U.S. Pat. No. 5,190,619 Berg et al. disclose the separation of 3-methyl-2-butanone from formic acid by extractive distillation with DMSO plus an additional compound. None of these references address the separation of inorganic salts from DMSO or DMSO/water/salt solutions.
Attempts to separate and recover DMSO from solutions of DMSO/inorganic salts/water or DMSO/inorganic salts by either thin film evaporators or vacuum distillation are not successful since, at even moderately elevated temperatures, DMSO and inorganic salts can react to decompose and contaminate the DMSO. Often the reaction is sufficiently exothermic that a dangerous condition results, possibly damaging the separation apparatus and endangering the system operator. Attempts to separate inorganic salts from DMSO/water solutions at lower temperatures by vacuum fractionation also encounter difficulties where the salt becomes insoluble and sticks to the surfaces of the apparatus used in the fractionation. The insoluble salt may collect to the point of plugging pipes and heat exchange equipment.
To overcome this problem in the recovery of DMSO from the above described solvent solutions, applicant has discovered a new process for separation and recovery of DMSO.